Our present invention relates to a method and apparatus for producing, by injection or compression molding (collectively referred to as pressure molding), partly finished workpieces which are transferred sequentially to one or more after treatment stations for the performance of secondary operations thereon. In particular, the workpieces may be parisons of polymeric material and the secondary operations may be a thermal conditioning thereof followed by their transformation into hollow articles, such as containers, by blowing with or witout mechanical stretching.
A system for conveying parisons or preforms by means of pallets into and out of a blow molding station has been described in U.S. Pat. No. 4,426,202 to Krishnakumar et al. According to that patent, the parisons or preforms presumably molded at a remote location are carried in groups on rotatable collets of roller-supported pallets which move on a track to the blow molding station where each parison is introduced between two open mold halves, subsequently closing therearound for transforming same into bottles. During the blow and briefly thereafter, the parisons and the bottles formed therefrom remain attached by their necks to the collets of their pallets which, however, do not otherwise participate in the blow molding operation. On the contrary, as expressly stated in the patent, the support of the preforms, i.e. the engagement of their flanges for holding them in position, is transferred from the pallet to the blow mold during the blowing operation. Nothing is said about where or how the pallets, which are separated from their track after the blow, are loaded and unloaded.
A major object of the present invention is to provide an improved method and apparatus for the continuous recirculation of a sufficient number of pallets between a receiving station, at which they are to be loaded with workpieces yet to be finished, and one or more aftertreatment stations for further handling and finishing.
Another object is to provide a method and apparatus as aforesaid in which the loading of the pallets with newly arriving workpieces, their transportation to the aftertreatment stations, their handling in the latter stations and the return of the pallets to the receiving station are precisely synchronized with one another and with the source of the workpieces, e.g., an injection molding machine, to provide the desired high degree of efficiency.
It is also an object of the present invention to provide an improved pallet structure for a system of this type which enables accurate location and secure retention of each parison in a finishing station, particularly in a blow molding station in which it is inflated with or without the use of a stretching rod.
A more particular object of the present invention is to synchronize the operation of a molding apparatus, specifically an injection molding machine, with those of a temperature conditioning and a blow molding station, having regard to the fact that an operating cycle of the latter station is considerably shorter than that of an injection molding machine so that the difference in timing has to be taken into account. Since the overall number of available pallets will normally be limited, the present invention further aims at insuring a return of unloaded pallets to the loading point in time for having them receive the newly arriving partly finished workpieces in keeping with the cycle of the injection molding machine.